Many scientific endeavors, especially space exploration and astronomical measurement applications, require highly sensitive electromagnetic radiation detectors, such as photon detectors. Charge-coupled detectors (CCDs) provide high quantum efficiency, broad spectral response, low readout noise, and high resolution. Therefore, CCD devices have been used extensively in scientific applications.
To be effective for most scientific applications, CCD devices must exhibit nearly perfect charge-transfer efficiency. Therefore, CCD devices are fabricated using specialized processes, such as the buried channel and peristaltic fabrication processes, which leave little, if any, imperfections in the semiconductor materials from which the CCD devices are formed. The CCD devices produced by these processes are generally characterized by high power consumption.
Other imaging devices, such as charge injection devices (CIDs) and active pixel sensors (APSs), are formed using conventional complementary metal-oxide semiconductor (CMOS) processes. CMOS devices typically exhibit much lower power consumption than CCD devices. Moreover, CMOS fabrication processes allow other imaging components, such as signal-processing circuitry, to be formed on the detector chip. As a result, CMOS imagers are preferred over CCDs in some scientific applications. The use of CMOS imagers has been limited, however, by characteristically low quantum efficiencies and high levels of fixed pattern noise in captured images.
In general, the fabrication processes used to produce CCD and CMOS imagers are incompatible. Conventional CMOS fabrication processes occur, at least in part, at temperatures that produce imperfections in the underlying semiconductor materials. While generally acceptable in CMOS devices, these imperfections typically reduce the efficiency of CCD devices to unacceptable levels.